ABSTRACT Shared epitope (SE)-coding HLA-DRB1 alleles confer the single strongest genetic risk for severe RA. In addition to genetic predisposition, exposure to environmental pollutants, such as dioxin-like compounds, and tobacco smoke strongly affect RA risk and severity. Furthermore, interactions between SE and tobacco smoke exposure has been shown to increase the risk for RA in a multiplicative, dose-dependent fashion. The mechanistic basis of this epidemiologically observed gene-environment interaction is unknown. We have recently uncovered an NF-kB-mediated crosstalk between the SE and aryl hydrocarbon receptor (AhR) pathways that lead to synergistic effects on osteoclast differentiation and Th17 polarization in vitro. Administration of AhR pathway agonists to transgenic mice carrying human SE-coding alleles resulted in a robust increase in arthritis severity, bone destruction and overabundance of osteoclasts and IL17-expressing cells in the inflamed joints and draining lymph nodes of arthritic mice. Thus, we have uncovered a previously unrecognized mechanism of gene-environment interaction. The studies proposed here will focused on detailed characterization of the newly uncovered synergism by defining the transcriptomic effects of the SE ligand, AhR agonists, and their combination. The physiologic relevance of the findings will be corroborated in in vivo mouse systems, as well as by human translational studies. The proposed research will have the following specific aims: ? To identify and map the synergistic pathways using an RNA-seq approach ? To determine the effect of the interaction on chromatin accessibility and validate transcription factor activation ? To validate interacting pathways in mice exposed to AhR agonists ? To determine pathway interactions in human cell lines and primary cells When successfully completed, the proposed project will have identified new pathogenic mechanisms and disease risk and severity markers in RA. Such findings will open the door to identification of new therapeutic targets that could be pursued in future research projects.